This opinion crops up from time to time…
There are some very debatable points made here. For example it assumes that the end destinations of many passengers travelling inbound on the Great Western Main Line lies on a east-west axis through Paddington and that Paddington is further from ‘the centre’ of London than other terminii.
It certainly depends - if people are taking the tube I'd say anecdotally it seems more are taking the Bakerloo which suggests more C. London destinations but of course it could be further beyond. I'm not personally of the impression it's that far from the "centre" but some do.
The stations in Crossrail’s core tunnels directly serve a corridor a few hundred metres from the station entrance - a long thin corridor corresponding to that of the Central Line. There are no car parks, only some Tubes and buses for destinations a bit furthr away. It offers no advantages to people wanting to reach Richmond (as there will be no convenient connection at OOC for decades into the future), Wimbledon, Wembley, Victoria or anywhere along the Euston Road corridor. Many of the London landmarks are not very close to the Crossrail stations either - for all its whizzy technology Crossrail and Tube stations in Zone 1 generally serve destinations within walking distance only.
That's an odd comment and you could say a lot about catchment areas of any major tube station; I'm not sure how exiting a physical station can serve anything but "destinations within walking distance only". Many of the core stations are already big and have bus connections or landmarks nearby. It's certainly going to be a very fast way to get into C. London and also connect to airports for instance. Whilst it is odd there's no connection to the Victoria line, many other places are a simple change away. I'm sure any stations or parts of London could be cherry-picked as "no advantages"
Changing to Crossrail at OOC for passengers on the longer distance trains whose destination is ‘London’ gives no advantage and some disadvantages; a stop will slow down journeys for all those that want other tube lines, taxis for destinations in Zone 1, parts of Zone 2 and buses or even walking to one of the many new offices along the Paddington Basin of the Grand Union Canal. It will be just as easy to change at Paddington for Crossrail, it is where the old cab road for setting down passengers for Departures used to be.
It may well add some benefit if it is permissible to change and go to inner W. London or Heathrow. I'd see OOC as similar to how Clapham Junction or London Bridge (less so as its more central) operates. There might be a small number of trains that don't call if that is deemed suitable. I sincerely doubt it will be as easy to change at Paddington given the number of levels you would need to descend and negotiate to reach the concourse but that will be hard to test until both are open.
What Crossrail will do, as the RER did in Paris and the S-Bahn in Munich, is to bring areas on each side of the central zone closer to each other. Crossing the centres of these and other cities was always slow as the Underground, Métro or U-Bahn have frequent stops as do buses and these are also affected by other road traffic. It has its own equivalent of Paris’s Châtelet/Les Halles at Farringdon where RER-type lines meet. Essentially it is a metro operation and the benefits are mainly within the city.
I don't disagree that it is similar to the RER and I can't comment on whether that had an impact on passengers interchanging at places like Gare Du Nord or Gare du Lyon although it would be interesting to know. It's not as simple as being a metro service and is in many ways inner/outer suburban.
I also struggle with the concept of ‘connectivity’ at OOC. There are two distinct traffic flows on the GW: long(er) distance from Swansea and Cardiff, Penzance and Plymouth, Cheltenham, Worcester, Oxford and Bristol on the one hand and the Thames Valley local services including Crossrail to Heathrow and Reading on the other. The local services will all go through the Crossrail core and passengers on the (Oxford)-Didcot-Paddington services will have a same platform interchange at the station of their choice.
Yes I think it might be interesting how somewhere like Ealing Broadway would have same platform changes with a Crossrail service. That doesn't detract from the overall argument that they still wouldn't be changing at Paddington like now (although many connect with the Central lines at Ealing Broadway or Stratford anyway today).
The issues surrounding connections from HS2 to Crossrail and the GW services were debated here recently so there is nothing really to add. Suffice it to say that there is a lot of sense for HS2 to connect to the Heathrow services and the Thames Valley local services but I was unconvinced that there is a sufficiently large Plymouth, say, to Birmingham flow to justify stopping all the GW Main Line services at OOC. Some people obviously make this journey but I have not been convinced that even if 10 or so people change at OOC off each train it is justified by lengthening the journey time of the other 400 people by 4 or 5 minutes.
It is difficult to know how passenger flows will alter and I agree it has been heavily debated before. The fact of the matter is that people who are going to London are going to all sorts of places as you rightly point out and will OOC help them get their quicker; I don't think it obviously has as many options as Clapham Junction or Stratford, but it may well become a "Western" hub over time and is set to become a rather large destination in its own right if you look at the plans for Old Oak & Park Royal. As as result, it seems reasonable all trains call there and in fact it might be practical for a "all or nothing" approach anyway.
Any capacity released at Paddington by the removal of the local services from the train shed will enable the longer distance offering to be improved: examples are a more frequent and regular semi-fast service down the Berks & Hants to Westbury or Exeter, the introduction of a new service west throught the Vale of the White Horse as a semi-fast towards Bristol calling at the long-proposed stations at Grove (for Wantage), Uffington, Shrivenham, Wootton Basset, Box and Corsham for which paths and Terminal platform capacity is non-existent at the moment. Other suggestions exist. This is much more likely that spending millions to get some of the existing Chiltern services running into Paddington - as the Chiltern journeys are so much shorter any additional fare income is highly unlikely to service the debt.
I too think the Chiltern option is something of a red herring (although it would be interesting to see the best use of the NNL - I personally think a dedicated cycle lane on the disused track to Old Oak Common might be an interesting option). The fact remains that just because Crossrail is going to run through, it releases NO capacity on the main or relief lines (indeed the service is meant to be more intense). Only Paddington/Liverpool St see an uptick in capacity.